Basic steps in triangulation. It takes 3 people. 1 to find coordinates on the groundplan and 2 to do actual measuring and taping.

Find 2 points at opposite sides of your groundplan. (Proscenium R and L works well)

Mark those points on your floor. Attach a long tape measure to each point.

Using a scale rule, find the coordinates of the corners of the objects you want to draw. (10'3" from Prosc L and 9'4" from Prosc R. Just measure in a straight line from reference points to corners, you will be making a triangle)

Then using your long measuring tapes find where 10'3" on the left one intersects 9'4" on the right one. Where they line up, is corner of your triangle and the corner of whatever object you are trying to tape out.

It is best to do the paperwork in advance and have all your coordinates mapped out ahead of time so you can just call the numbers to your tapers.

I used to use triangulation a lot in college for taping groundplans. Now I most use more of a grid system with a tape measure on center line and measuring out to the sides to find points.

I DO however still use triangulation to find USC. DSC is usually easy (just go off architecture), but USC can be tricky. So I will have people hold the tape measures at proscenium corners and then find where same numbers intersect along the line that I want my mark. Viola, accurate USC.